Heat exchangers of the type used to recover heat from gas turbine or gas/diesel engine exhaust gas are commonly designed with a bypass circuit situated external to the heat exchanger array and its casing, with the exhaust gas flow to the heat exchanger array circuit and the bypass circuit controlled by at least one flap valve or the like, such valves being known as dampers.
DE 42 07 667 shows a heat exchange unit having a damper in the form of a sleeve valve axially slideable within a casing to open or close the entry to a duct containing a heat exchanger. When the sleeve valve closes the heat exchanger, exhaust gas bypasses the heat exchanger and passes exclusively through the sleeve. However, when the sleeve valve opens the duct, the exhaust gas passes through both the duct and the sleeve.
WO99/64806 also shows a heat exchange unit having a damper in the form of a slideable sleeve. However, the damper is arranged such that, at extreme positions thereof, exhaust gas can be selectively arranged to flow either through a bypass containing a heat exchanger or through a duct bypassing the heat exchanger. However, turbulence at downstream portion of the bypass duct can cause the gas to heat the heat exchanger even though gas is being diverted past the heat exchanger. Moreover, when the gas is being passed into the heat exchanger, the angle through which the gas is caused to move can create a significant pressure drop which can lead to an increased back pressure within the heat exchange unit. The skilled person will appreciate that if the back-pressure becomes too large then an engine feeding the heat exchange unit may not be as efficient as desired. Also, the particular arrangement of the damper means that the damper cannot be tested until the whole unit is assembled which can add to the time to manufacture the unit.
Another damper arrangement is provided in EP1923545A2, where the substantially rigid component surfaces of the damper are triangular in order that they may selectively mesh to seal the bypass duct and separate to seal the heat exchange duct. The disadvantage with this system is that for this damper to work the fluid must flow through triangular holes in the inlet duct to enter the heat exchange duct. The remaining casing surrounding the triangular holes therefore impedes the flow of gas to the heat exchanger even when the damper is positioned to allow gas to enter the heat exchanger.